Many CVT machines have over the years been proposed and developed in attempts to produce, from a constant input velocity, an acceptably linear continuously variable output angular velocity and torque.
The basic approach to these machines has initially been to apply a single input angular velocity to a number of parallel stages of the machines which, by means of angular velocity generators such as cams, slotted plates, linkages, off-centred sun gears, non-circular gears, orbital devices, universal joints, cranks, cams and so on to produce pulsed or cycled angular velocities (angular acceleration and deceleration cycles) in each of the stages which are appropriately phased to the input angular velocity. The pulsed or cycled angular velocities from the machine stages are then used to drive selective extraction devices, such as overrunning or sprag clutches and/or differentials which extract the maximum portions of the driven cycles of the parallel stages to provide modulated angular velocity outputs. These outputs are then recombined in an output stage of the machine to provide the output angular velocity of the machine.
Examples of such machines, particularly those in connection with cam based CVT's, are disclosed in the specifications of the following U.S. Pat. No. 6,425,301, U.S. Pat. No. 4,487,085 and U.S. Pat. No. 2,159,739.
Common problems which are typically associated with the above-mentioned CVT machines of the prior art are that:
the cam followers are in a single point contact with the cam, with the point contact generating unacceptably high stresses which make their use, in high power applications such as automotive or truck applications, impractical. This problem is partially solved by the disclosure in U.S. Pat. No. 2,159,739 by having a stepped cam rotor on which, in each rotor step, line contact with cam followers is maintained, but in transition from one step to the next involves point contact of the followers with the cam and therefore only partially solves the problem and then only in respect of discreet step fixed gear ratio machines.
In U.S. Pat. No. 6,425,301 an attempt is made to solve the point contact problem by employing a perfectly circular cam, the eccentric offset of which is adjustable by a repositioning of the axis of rotation of the cam. The circular cam profile of this type of arrangement creates an output angular velocity which is rippled and results in unacceptable vibration.
The cam shaped rotor profiles used in the prior art machines are incapable of providing a specifically defined linear scaling over the length of the rotor to make line contact with the cam followers possible.
Further in the prior art machines, the cam followers are attached to an oscillating shaft creating a crank or lever action with the follower attached to the end of the lever or crank while riding on the cam. This lever or crank action does not represent a linear conversion of cam lift to a rotational motion of the oscillating shaft as the linear motion of the cam is not connected in a linear fashion to the angular motion of the lever or crank, and therefore their constant cam lift rates do not result in a constant angular velocity of the oscillating shaft. To overcome this problem in some prior art applications the cam lift has been altered to produce a non-constant lift to compensate for the non-linear crank or lever conversion.
Other prior art machines employ the use of gears as well as rack and pinion arrangements to convert linear motion to rotational motion which results in high stresses at the gear interfaces and, with the crank/lever arrangements in practice being very short, results either in very high stresses or very large gear/rack and pinion arrangements in high power applications. None of the prior art machines provide for power transmission in both directions (are not able to perform engine braking) due to the use of sprag clutches and the inability of the followers to drive the cam in the cam lift stroke.
The general state of the prior art reflects inventions for use in low power applications, such as bicycles, where constant angular output velocity is not crucial.
The prior art machines are also not suitable for high power high speed applications in which the CVT's need to provide a very high consistency in output angular velocity to avoid vibration and are not adequately compact with minimal stress concentrations to make them economically viable in the transmission industry.